Key Points Question Is SARS-CoV-2 messenger RNA (mRNA) vaccination associated with risk of myocarditis? Findings In a cohort study of 23.1 million residents across 4 Nordic countries, risk of myocarditis after the first and second doses of SARS-CoV-2 mRNA vaccines was highest in young males aged 16 to 24 years after the second dose. For young males receiving 2 doses of the same vaccine, data were compatible with between 4 and 7 excess events in 28 days per 100 000 vaccinees after second-dose BNT162b2, and between 9 and 28 per 100 000 vaccinees after second-dose mRNA-1273. Meaning The risk of myocarditis in this large cohort study was highest in young males after the second SARS-CoV-2 vaccine dose, and this risk should be balanced against the benefits of protecting against severe COVID-19 disease.

The emergence of SARS-CoV-2 Omicron variant (B.1.1.529) with major spike protein mutations has raised concern over potential neutralization escape and breakthrough infections among vaccinated and previously SARS-CoV-2-infected subjects. We measured crossprotective antibodies against variants in health care workers (HCW, n = 20) and nursing home residents (n = 9) from samples collected at 1-2 months, following the booster (3rd) dose. We also assessed the antibody responses in subjects infected before the Omicron era (n = 38) with subsequent administration of a single mRNA vaccine dose. Following booster vaccination, HCWs had high IgG antibody concentrations to the spike protein and neutralizing antibodies (NAb) were detectable against all variants. IgG concentrations among the elderly remained lower, and some lacked NAbs against the Beta and Omicron variants. NAb titers were significantly reduced against Delta, Beta, and Omicron compared to WT virus regardless of age. Vaccination induced high IgG concentrations and variable titers of cross-reactive NAbs in previously infected subjects, whereas NAb titers against Omicron were barely detectable 1 month postinfection. High IgG concentrations with cross-protective neutralizing activity were detected after three Coronavirus Disease 2019 (COVID-19) vaccine doses in HCWs. However, lower NAb titers seen in the frail elderly suggest inadequate protection against Omicron breakthrough infections, yet protection against severe COVID-19 is expected.

ImportanceSpontaneous adverse reaction reports of sudden hearing loss have been observed, and a population-based cohort study conducted in Israel showed an increase in the incidence of sudden sensorineural hearing loss (SSNHL) following vaccination with messenger RNA COVID-19 vaccine BNT162b2 (Pfizer-BioNTech). However, in this setting, the possibility of confounding remained.ObjectiveTo assess a potential association between COVID-19 vaccinations and SSNHL.Design, Setting, and ParticipantsThis register-based country-wide retrospective cohort study of 5.5 million Finnish residents was conducted from January 1, 2019, to April 20, 2022, and included all individuals who were identified from the population information system who were alive or born during the study period except individuals who had SSNHL during 2015 to 2018 according to specialized care derived diagnosis codes for SSNHL (International Statistical Classification of Diseases and Related Health Problems, Tenth Revision [ICD-10] code H91.2) as a primary or secondary diagnosis.ExposuresThe a priori primary risk period was 0 to 54 days following each COVID-19 vaccination. The risk periods for different vaccine doses did not overlap so that a later vaccine exposure ended the previous risk period. The secondary risk period was from 55 days following each COVID-19 vaccination until a subsequent COVID-19 vaccination. A secondary analysis included a risk time from 0 to 54 days following a positive polymerase chain reaction test result for SARS-CoV-2.Main Outcomes and MeasuresThe incidences of SSNHL following COVID-19 vaccination were compared with the incidences before the COVID-19 epidemic in Finland. The Poisson regression model included calendar time, age, sex, diabetes, cardiovascular disease, other chronic diseases, and the number of visits in primary health care.ResultsFor the 5.5 million Finnish residents included in the study, the comparison time comprised 6.5 million person-years, the primary risk time of 1.7 million person-years, and the secondary risk time of 2.1 million person-years. Before the COVID-19 epidemic in Finland, 18.7/100 000 people received a diagnosis of SSNHL annually. The study data suggested no increased risk for SSNHL following any COVID-19 vaccination. In particular, adjusted incidence rate ratios with 95% confidence intervals for the BNT162b2 vaccine’s 3 doses were 0.8 (95% CI, 0.6-1.0), 0.9 (95% CI, 0.6-1.2), and 1.0 (95% CI, 0.7-1.4), respectively. There was no association between SARS-CoV-2 infection and an increased incidence of SSNHL.Conclusions and RelevanceThe results of this cohort study show no evidence of an increased risk of SSNHL following COVID-19 vaccination. The study accounted for previous disease and other potential confounding factors. These results are based on diagnosis codes in specialized care but still need to be verified in settings that are capable of evaluating the degree of hearing loss.

In Finland, the first wave of the COVID-19 epidemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) took place from March to June 2020, with the majority of COVID-19 cases diagnosed in the extended capital region. The magnitude and trend in the incidence of COVID-19 is one way to monitor the course of the epidemic. The diagnosed COVID-19 cases are a subset of the infections and therefore the COVID-19 incidence underestimates the SARS-CoV-2 incidence. The likelihood that an individual with SARS-CoV-2 infection is diagnosed with COVID-19 depends on the clinical manifestation as well as the infection testing policy and capacity. These factors may fluctuate over time and the underreporting of infections changes accordingly. Quantifying the extent of underreporting allows the assessment of the true incidence of infection. To obtain information on the incidence of SARS-CoV-2 infection in Finland, a series of serological surveys was initiated in April 2020. We develop a Bayesian inference approach and apply it to data from the serological surveys, registered COVID-19 cases, and external data, to estimate the time-dependent underreporting of SARS-Cov-2 infections during the first wave of the COVID-19 epidemic in Finland. There were 1 to 5 (95% probability) SARS-CoV-2 infections for every COVID-19 case during the first wave of the COVID-19 epidemic in Finland. The underreporting was highest before April when there were 4 to 17 (95% probability) infections for every COVID-19 case. It is likely that between 0.5%-1.0% (50% probability) and no more than 1.5% (95% probability) of the population in the extended capital region were infected with SARS-CoV-2 by the beginning of July 2020.